Camshaft phasers, as are known in the art, are used to control the angular relationship of a pulley or sprocket of a crankshaft of an internal combustion engine to a camshaft of the internal combustion engine. The camshaft phaser allows changing the phase relationship of the crankshaft and camshaft while the engine is running. Typically, the camshaft phaser is used to shift an intake camshaft on a dual overhead camshaft engine in order to broaden the torque curve of the engine, to increase peak power at high revolution speeds, and to improve the idle quality. Also, an exhaust camshaft can be shifted by another camshaft phaser in order to provide internal charge dilution control, which can significantly reduce HC and NOx emissions, or to improve fuel economy. The above objectives are in the following briefly termed as combustion demands. With this definition, the camshaft phaser is used to account for combustion demands.
Camshaft phasers are commonly controlled by hydraulic systems which use pressurized lubrication oil from the engine in order to change the relative phase relationship between the camshaft and the crankshaft, thus altering the valve timing. An advance or retard position of the camshaft is commanded via an oil control valve. The oil control valve controls the oil flow to different ports entering a camshaft phaser, thus controlling the angular position of the camshaft relative to the pulley or sprocket of the crankshaft. However, the efforts in the valve train may pressurize the oil contained in the chambers of the camshaft phaser such that the oil pressure inside the camshaft phaser reaches peaks which can be higher than the oil control supply pressure, i.e., the oil pressure supplied by the engine. This can lead to a certain amount of reverse oil flow across the oil control valve, thereby diminishing the phase rate performance of the camshaft phasing system.
To avoid the reverse oil flow under the above mentioned circumstances, recent approaches have proposed to employ a check valve integrated in the oil passage of either the cylinder head, crankcase, camshaft phaser, or a manifold. Such a check valve also ensures that the camshaft phaser does not empty out in cases when the oil pressure is reduced, for example when the engine is stopped. However, this approach adds significant cost to the cylinder head, engine block, camshaft phaser, or manifold. Additionally, the implementation of the check valve can be difficult because of oil routing and the check valve may add an undesired restriction to the oil passage. Adding restriction may require the use of an oil pump larger than would otherwise be required, thereby decreasing the fuel efficiency of the internal combustion engine. Furthermore, the check valve should not be placed too far away from the camshaft phaser in order to remain effective. While some camshaft phasing systems have integrated a check valve directly within the camshaft phaser in order to maximize the effectiveness of the check valve, space within the camshaft phaser can be extremely limited, thereby making integration of the check valve within the camshaft phaser difficult.
U.S. Pat. No. 7,584,728; commonly assigned to Applicant and incorporated herein by reference in its entirety; teaches a strategy for controlling the oil control valve to avoid the reverse oil flow caused by efforts of the valve train while the internal combustion engine is running and without using a separate check valve. In this strategy, a spool of the oil control valve is synchronized to block ports when valve train efforts produce oil pressures within the camshaft phaser that are higher than oil pressure being supplied to the camshaft phaser from the oil source. In this way, a separate check valve is not needed in order to avoid the reverse oil flow while the internal combustion engine is operating. While this control strategy solves the problem of reverse oil flow while the engine is running, reverse oil flow may still occur when the engine is not running because the default position of the spool of the oil control valve provides fluid communication between the camshaft phaser and the oil source as well as between the camshaft phaser and a vent.
As an effort to conserve fuel, the internal combustion engine of some motor vehicles is automatically turned off, rather than allowing the internal combustion engine to idle, when the motor vehicle comes to a stop, for example, when the motor vehicle is stopped at a traffic light. This event may be known as automatic stop mode because the operator of the internal combustion engine has not turned off the ignition to the motor vehicle and various subsystems operate on battery power in anticipation of a near-term restart of the internal combustion engine. The internal combustion engine is then automatically restarted when propulsion is again desired which may be determined, for example, by the operator of the motor vehicle removing their foot from the brake pedal or applying pressure to the accelerator pedal. If such a motor vehicle uses the strategy of U.S. Pat. No. 7,584,728 to control the oil control valve rather than using a separate check valve, oil pressure prime may be lost in the camshaft phaser each time the internal combustion engine is in automatic stop mode. This may be undesirable, for example, because camshaft phasing may not be available until sufficient time has been allowed to elapse after the internal combustion engine has been restarted in order to allow sufficient time to replenish oil to the camshaft phaser. The camshaft phaser may also produce an objectionable audible noise if pressure prime has been lost.
As another effort to conserve fuel, some motor vehicles, commonly referred to as hybrid electric vehicles, may be equipped not only with an internal combustion engine, but also with an electric motor which receives power from electricity stored in a battery. The hybrid electric vehicle may be propelled solely by the electric motor when propulsion demands can be met without the internal combustion engine. However, the internal combustion engine can be started automatically when propulsion demands so require. When the hybrid electric vehicle is propelled solely by the electric motor, the internal combustion engine is in an automatic stop mode just as in the previous example because the internal combustion engine can be started automatically as needed. Hybrid electric vehicles may suffer the same drawbacks as in the previous example if the strategy of U.S. Pat. No. 7,584,728 to control the oil control valve is used rather than using a separate check valve.
What is needed is a method to avoid reverse oil flow from a camshaft phaser when an internal combustion engine is temporarily not running and which does not require a mechanical check valve.